One of the hallmarks of the adaptive response to hypoxia is activation of the transcription factor Hypoxia Inducible factor 1alpha (HIF-1alpha). Hypoxic zones exist in the mammalian fetal growth plate during important junctures of chondrocyte morphogenesis. Conditional inactivation of HIF-1alpha has demonstrated that this transcription factor is critical for growth arrest and survival of growth plate chondrocytes in vivo. In this grant proposal, we will explore the role of hypoxia in regulating chondrocyte proliferation, differentiation and death. In Specific Aim I we will address the question whether the action of HIF-1alpha as a "survival factor" in the mammalian growth plate is exclusively cell-autonomous (i.e. through up-regulation of metabolic pathways such as anaerobic glycolysis), or whether it requires also non cell-autonomous mechanisms, such as up-regulation of VEGF and angiogenesis, by studying a chimeric growth plate in which only a small percentage of the chondrocyte population lacks the HIF-1alpha gene. In Specific Aim II we will perform a systematic study in vitro of the role of hypoxia in chondrocyte activity, by analyzing in vitro wild type chondrocytes and chondrocytes lacking the HIF-1alpha gene in both normoxic and hypoxic conditions. In Specific Aim III we will explore in vivo the consequences of increased HIF-1alpha protein levels resulting from conditional inactivation of the von Hippel Lindau gene (VHL), a critical negative regulator of HIF-1alpha protein stability, and we will establish the importance in vivo of the ubiquitylation pathway in modulating HIF-1alpha transcriptional activity. The fetal growth model is a useful in vivo model to study the biological role of hypoxia in modulating cell proliferation, differentiation and death. The study outlined above will provide novel insights into both chondrocyte biology and mechanisms of action of hypoxia, HIF-1alpha and VHL.